Downhole heterogeneous proppant

ABSTRACT

A technique facilitates treatment of a subterranean formation. A proppant and a proppant carrier fluid are delivered to a subterranean location for treatment of the formation. At the subterranean location, heterogeneities of proppant structures are generated with the proppant and the proppant carrier fluid. The heterogeneous proppant structures are then transported into the subterranean formation to improve conductivity.

BACKGROUND

In producing oil and gas, a variety of subterranean geologic formations lack sufficient permeability for optimal production of the hydrocarbons. The low permeability reduces the potential production rate of the hydrocarbon fluids. However, the flow rate can be increased by performing stimulation treatments, such as hydraulic fracturing, on the formation. By way of example, hydraulic fracturing may be performed by hydraulically injecting a fracturing fluid at high pressure, e.g. in excess of 10,000 psi, into the wellbore and ultimately into the surrounding formation. Once the pressure exceeds a threshold value, the formation strata/rock fractures and the fracturing fluid propagates into the formation. The fracturing fluid carries proppant particles into the extending fractures, and the proppant particles are deposited in the created fractures to prop open the fractures. By delivering the proppant into the fractures, the potential flow of recoverable fluid is improved although the homogeneous mixture of proppant in the fracturing fluid limits the improvement. The homogeneous matrix of packed proppant affects the fracture conductivity which is the ability of fluids to flow from the formation, through the matrix of packed proppant, and into the production wellbore.

Various methods have been employed for controlling the proppant pack permeability in an effort to enhance hydraulic conductivity. For example, U.S. Pat. Nos. 3,592,266; 3,850,247; 5,411,091; 6,776,235; 7,213,651; and 7,451,812 propose high conductivity channels by pumping alternating intervals of fracturing slurries which are different in at least one of their parameters. Many of these techniques assume that heterogeneity introduced at an early stage of hydraulic fracturing treatment will be preserved throughout the treatment process. However, one of the main problems in creating heterogeneities of proppant structures at the surface when fluids are mixed and pumped into the wellbore is a homogeneous dispersion of the heterogeneities upon arriving at the perforation or fracture.

SUMMARY

In general, the present disclosure provides a system and method for use in treating a subterranean formation. A proppant and a proppant carrier fluid are combined into a slurry and delivered to a subterranean location to facilitate treatment of the formation. At the subterranean location, heterogeneities of proppant structures are generated with the proppant and the proppant carrier fluid. The heterogeneous proppant structures are then transported into the subterranean formation to greatly improve conductivity.

However, many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein, and:

FIG. 1 is an illustration of an example of a system, e.g. a well system, deployed to deliver a treatment to a subterranean formation, according to an embodiment of the disclosure;

FIG. 2 is a schematic illustration of an example of a tool positioned at a downhole location and comprising a mechanism to create heterogeneous proppant structures from proppant and the proppant carrier fluid received by the tool, according to an embodiment of the disclosure;

FIG. 3 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 4 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 5 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 6 is a schematic illustration similar to that of FIG. 5 but showing the tool in a different operational position, according to an embodiment of the disclosure;

FIG. 7 is a schematic illustration similar to that of FIG. 6 but showing the tool in a different operational position, according to an embodiment of the disclosure;

FIG. 8 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 9 is a schematic illustration similar to that of FIG. 8 but showing the tool in a different operational position, according to an embodiment of the disclosure;

FIG. 10 is a schematic illustration similar to that of FIG. 9 but showing the tool in a different operational position, according to an embodiment of the disclosure;

FIG. 11 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 12 is a cross-sectional top view of the tool illustrated in FIG. 11, according to an embodiment of the disclosure;

FIG. 13 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 14 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 15 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 16 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 17 is an enlarged view of the tool illustrated in FIG. 16, according to an embodiment of the disclosure;

FIG. 18 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 19 is a schematic top view of the tool illustrated in FIG. 18, according to an embodiment of the disclosure;

FIG. 20 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 21 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 22 is a schematic illustration of a spiraling concentration of proppant material distributed by the tool illustrated in FIG. 21, according to an embodiment of the disclosure;

FIG. 23 is a schematic illustration of proppant structures distributed into a surrounding formation in a generally concentric orientation, according to an embodiment of the disclosure;

FIG. 24 is a schematic illustration of another example of a portion of a tool used to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 25 is a schematic illustration of another example of a portion of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure;

FIG. 26 is a schematic illustration of another example of a tool to create heterogeneous proppant structures at a subterranean location, according to an embodiment of the disclosure; and

FIG. 27 is a schematic illustration of proppant structures distributed into a surrounding formation in a generally perpendicular orientation with respect to the well, according to an embodiment of the disclosure.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.

The disclosure herein generally involves a system and methodology for treating a subterranean formation. For example, the system and methodology may be employed to facilitate a fracturing operation with respect to a subterranean formation surrounding a wellbore. In a variety of fracturing or other treatment applications, a proppant and a proppant carrier fluid are delivered to the subterranean location to facilitate treatment of the formation. When employed in fracturing, the proppant and proppant carrier fluid may be combined into a slurry to form a variety of fracturing fluids. At the subterranean location, heterogeneities of proppant structures are generated with the proppant and the proppant carrier fluid. For example, a subterranean tool, such as a bottom hole tool located in a wellbore, may be employed at the subterranean location to create heterogeneous proppant structures. The heterogeneous proppant structures are then transported into the subterranean formation to improve conductivity.

Fracturing fluids may comprise a variety of materials, such as proppant and removable proppant-spacing material. The proppant-spacing material may be designed to function in forming open channels and spaces around clusters of proppant. Such extramatrical channel-forming materials, including proppant-spacing particles, are sometimes referred to as channelant.

In some applications, the term “proppant” may be employed to describe materials comprising channelant and sized particles which may be mixed with a proppant carrier fluid to help provide an efficient conduit for production of fluid from the formation/reservoir to the wellbore. Proppant may comprise naturally occurring sand grains or gravel, man-made or specially engineered proppants, e.g. resin-coated sand, or high-strength ceramic materials, e.g. sintered bauxite. Proppant materials also may comprise fibers, such as fibers formed from glass, ceramics, carbon (including carbon-based compounds), metal (including metallic alloys), polymeric materials (e.g. PLA, PGA, PET, polyol) and other materials or combinations of such materials.

The proppant also may be formed in a variety of sizes or ranges of sizes of material having mixed shapes, variable diameters, or other properties that yield, for example, high-density and high-strength properties to increase fracture conductivity. By way of specific examples, hydraulic fracturing can use up to 50 tons or more of proppant in which 10-15 tons have particle diameters from 0.002 to 0.1 mm; 15-30 tons have particle diameters from 0.2 to 0.6 mm; and 10-15 tons have particle diameters from 0.005 to 0.05 mm. Proppant size, however, may vary from job to job and from stage to stage. In some examples, the proppant comprises particles having an average particle size of from about 0.15 mm to about 2.5 mm; and additional examples of size ranges are from about 0.25-0.43 mm, 0.43-0.85 mm, 0.85-1.18 mm, 1.18-1.70 mm, and 1.70-2.36 mm.

As described in greater detail below, the proppant is used in a system and methodology for generating heterogeneities of proppant structures at a subterranean location (in situ) during a treatment operation, e.g. during a perforation/hydraulic fracturing operation. In a variety of well applications, the system and methodology may be used in stimulating, e.g. hydraulically fracturing, a subterranean formation penetrated by a wellbore combined with placing propping agents in a fracture heterogeneously to further enhance conductivity.

Referring generally to FIG. 1, an example of a subterranean treatment system is illustrated as comprising a well system having a tubing string deployed in a well. The well system can be used in a variety of well applications, including onshore applications and offshore applications. In this example, the tubing string is illustrated as deployed in a generally vertical wellbore, however the tubing string may be deployed in a variety of wells including various vertical and deviated wells. The embodiments described below may be employed to facilitate, for example, fracturing operations in well applications and in other types of applications.

In the example illustrated in FIG. 1, a well system 30 is deployed in a wellbore 32 and comprises a tubing string 34 having at least one flow path 36. In the example illustrated, a plurality of flow paths 36 is represented by arrows 38 and may be created by tubular structures 40. A proppant delivery system 42 delivers a proppant 48 and proppant carrier fluid 50 in the form of a slurry, down along tubing string 34 to a tool 44. In a fracturing application, for example, fracturing fluid may be delivered down through tubing string 34 and into the tool 44 which may be positioned at a bottom hole end of the tubing string 34. In some applications, the tubing string 34 comprises a slurry line, e.g. a coiled tubing slurry line, and tool 44 is coupled to the slurry line at its lower end. Tool 44 comprises a mechanism 46 to generate heterogeneities of proppant structures at the downhole location during a treatment operation, such as a fracturing operation.

During a fracturing operation, fracturing fluid is pumped down along tubing string 34 and may comprise proppant 48 and proppant carrier fluid 50, e.g. a clean fluid/lower proppant concentration fluid, which carries the proppant 48. Depending on the parameters of a given application, the proppant 48 and proppant carrier fluid 50 may be delivered downhole along a single flow path 36 or along a plurality of flow paths 36. The fracturing fluid/slurry comprising proppant 48 and proppant carrier fluid 50 is delivered to downhole tool 44, and mechanism 46 is employed to generate heterogeneous proppant structures 52 which have a different concentration of proppant than the surrounding fluid. For example, the proppant structures 52 may comprise higher concentrations of proppant 48.

The fracturing fluid, comprising the newly (and locally) formed heterogeneous proppant structures 52, is discharged by tool 44 into a surrounding formation 54 of a well zone 55 under sufficient pressure to create a fracture or fractures 56 in the formation 54. In the example illustrated, tool 44 disperses the fracturing fluid 48, 50 and its heterogeneous proppant structures 52 through one or more perforations 58 forming a perforation zone of the wellbore 32 communicating with the surrounding formation 54. The perforations extend outwardly through a wall forming wellbore 32 and into formation 54. Forming the heterogeneous proppant structures 52 in close proximity to the perforation zone at the downhole location helps maintain the heterogeneity as the material is moved into fracture 56, thus improving the conductivity of well fluid moving from formation 54, along fracture 56, and into wellbore 32.

Various embodiments of tool 44 may be used to implement the downhole generation of heterogeneities of proppant structures and to thus create highly conductive hydraulic fractures. Referring generally to FIGS. 2 and 3, certain embodiments of tool 44 may utilize centrifugal forces to generate the heterogeneous proppant structures 52. In the example illustrated in FIG. 2, tool 44 utilizes mechanism 46 in the form of a hydrocyclone 60 which may be active or passive. An active form of hydrocyclone 60 refers to a powered mechanism employing a motor or other motive source to spin the proppant 48 and proppant carrier fluid 50 in a manner that separates the proppant 48 and proppant carrier fluid 50 into streams having relatively higher concentration and lower concentration of proppant 48. A passive form of hydrocyclone 60 is one that uses the natural flow of the proppant 48 and proppant carrier fluid 50 to create the centrifugal forces separating the proppant 48 and proppant carrier fluid 50 into flow streams which may comprise different concentrations of proppant 48.

Regardless of whether the mechanism 46 is active or passive, the tool 44 illustrated in FIG. 2 receives the proppant 48 and proppant carrier fluid 50 along a single flow path, e.g. through a slurry line 62. The proppant 48 and proppant carrier fluid 50 is delivered to hydrocyclone 60 through an inlet 64 and is directed in a circular, e.g. spiral, pattern to induce centrifugal forces which separate the proppant 48 and proppant carrier fluid 50 into a higher concentration flow of proppant 48 which exits through an outlet 66 and a lower concentration flow of proppant 48 which exits through a second outlet 68. The flow path of the higher concentration flow is illustrated by arrow 70 and the flow path of the lower concentration flow is illustrated by arrow 72. In this particular example, tool 44 also may comprise a supplemental mechanism 74 designed to receive the higher concentration flow 70 and to separate the higher concentration flow into independent or separated heterogeneous proppant structures 52. Some examples of such mechanisms for generating independent heterogeneous proppant structures 52 are described in greater detail below.

As illustrated in the embodiment of FIG. 3, however, the heterogeneous proppant structures 52 may be created via a continual stream of concentrated proppant 48. In this example, at least one flow path 36 is used for delivering proppant 48 and proppant carrier fluid 50 to a perforation zone. The heterogeneous proppant structures 52 can again be generated by tool 44 in the form of hydrocyclone 60 located downhole at, for example, a bottom end of slurry line 62. In this example, the higher concentration flow 70 of proppant 48 is discharged through a nozzle or other suitable device 76 for distribution through perforations 58 and into fracture 56. The decomposition of the slurry flow into two flows is based on the centrifugal forces induced in tool 44. Homogeneous mixing of the higher concentration flow 70 with the lower concentration flow 72 is reduced or prevented by creating spiral slurry strings 78. Due to the close proximity of the perforation zone, the spiral slurry strings 78 remain unmixed before being divided by the perforations 58 into heterogeneous proppant structures 52. Consequently, the heterogeneous proppant structures 52 are flowed into the fracture 56 heterogeneously.

In this latter example, the downhole tool 44 may be used to decompose the slurry flow into two flows by utilizing different geometries of the tool. For example, tool 44 may be in the form of a hydrocyclone without rotating parts or in the form of a centrifugal pump having rotating parts. The spiral slurry strings may be created within a surrounding casing 80 by rotating outlet/nozzle 76 to create a spiral flow of the higher concentration material or by rotating the entire tool 44 along a vertical axis. Accordingly, the tool 44 may be an active tool using the energy of engines or other external sources, or the tool may be passive and use the energy of the flowing slurry. It should be noted that decomposition of the slurry flow also can be affected by the physical properties of the slurry, e.g. flow speed, slurry density, or other slurry characteristics.

Referring generally to FIG. 4, another embodiment of tool 44 is illustrated. In this embodiment, the proppant 48 and proppant carrier fluid 50 are again delivered downhole as a slurry 82 along single or plural flow paths 36. As illustrated, the proppant 48 and proppant carrier fluid 50 may be delivered along single flow path 36 within slurry line 62 to a downhole location below a packer 84. Packer 84 separates the slurry line 62 from the surrounding casing 80. In this embodiment, tool 44 accumulates the proppant 48 from the slurry 82 and allows the separated clean fluid to pass through the tool 44. The accumulated proppant 48 is released periodically in a given size, volume or mass into the separated clean fluid flowing down through perforations 58. The periodically released proppant 48 creates the heterogeneous proppant structures 52 which are carried down through perforations 58 and into fracture 56.

In this embodiment, the tool 44 may comprise a sieve 88 which is pivotably mounted within slurry line 62 for pivoting motion about an axis 90. The sieve 88 is designed to accumulate the proppant 48 from the slurry 82 while allowing the clean fluid to pass through until a desired amount of proppant 48 is collected. A trigger mechanism 92, e.g. motor, spring mechanism, releasable latch, or other suitable trigger mechanism, is then actuated to enable rotation of the sieve 88 to a position which dumps the proppant 48 to form a heterogeneous proppant structure 52. This process is repeated to create the plurality of heterogeneous proppant structures 52 routed into fracture 56.

In a similar example, the proppant 48 is accumulated along a plurality of plates or ledges 94, as illustrated in FIGS. 5-7. In this embodiment, the ledges 94 are located at different positions along the tool 44 such that the slurry 82 is forced to flow along a tortuous path 96 between the ledges 94 in a manner that causes proppant 48 to accumulate on the ledges 94, as illustrated in FIG. 5. A trigger device (e.g. see trigger mechanism 92 in FIG. 4) is employed to rapidly pivot the ledges 94 through, for example, 90° to release the accumulated proppant 48, as illustrated in FIG. 6. A spring mechanism 98 or other suitable mechanism may be used to return the ledges 94 back to their original positions to again begin accumulating proppant 48, as illustrated in FIG. 7. This process may be repeated to create the heterogeneous proppant structures 52 containing more highly concentrated proppant 48, as also illustrated in FIG. 7. The heterogeneous proppant structures 52 are carried through perforations 58 and transported into fracture 56.

The embodiments illustrated in FIGS. 4-7 may utilize a variety of configurations and tool geometries to generate the proppant structure heterogeneities. For example, tool 44 may comprise rotating sieves, meshes, or screens to accumulate the proppant 48. The tool 44 also may utilize fluid-induced vibrations or it may comprise self-opening assemblies to provide controlled release of proppant as heterogeneous proppant structures 52 in desired sizes, volumes or portions. Additionally, the tool 44 may utilize a variety of trigger mechanisms 92 for releasing proppant 48 in the form of heterogeneous proppant structures 52 based on pressure, mass of proppant, time of accumulation, or other triggering events. Additionally, tool 44 may be actively or passively operated by using an external source of energy or by using the energy of the flow, respectively.

Referring generally to FIGS. 8-10, another embodiment of tool 44 is illustrated. Similar to the previously described embodiments, the tortuous path 96 (or paths) is routed through tool 44 to direct the slurry 82 through sharp changes in direction. Due to the sharp changes of direction in the flowing slurry, proppant 48 is accumulated at elbows 100 formed along the tortuous path 96, as illustrated in FIG. 8. A trigger device (e.g. see trigger mechanism 92 in FIG. 4) is employed to rapidly pivot the elbows 100 in a manner which releases the accumulated proppant 48, as illustrated in FIG. 9. A spring mechanism 98 or other suitable mechanism may be used to restore the elbows 100 so as to again begin accumulating proppant 48, as illustrated in FIG. 10. This process may be repeated to create the heterogeneous proppant structures 52 containing more highly concentrated proppant 48 (see FIG. 10). The heterogeneous proppant structures 52 are flowed through perforations 58 and transported into fracture 56.

In the latter embodiment, the proppant structure heterogeneities 52 may be created by various tool configurations. For example, the tool 44 may be designed with different numbers and configurations of tortuous flow paths 96. The tool 44 also may contain various configurations, mechanisms and ports in different numbers to accumulate proppant 48 along the tortuous paths 96. The releasable elbows 100 also may be formed with ledges, plates, or other mechanisms designed to release heterogeneous proppant structures 52 of predetermined sizes, volumes, or portions. Several types of triggers 92 also may be employed to release the accumulated proppant 48 based on, for example, pressure, mass of proppant, time of accumulation, or other factors. Different types of flow paths 96 may be established through the tool 44 to accommodate different types of proppant 48.

In another embodiment, the tool 44 comprises a bottom hole tool which may receive slurry along individual or plural flow paths 36. In this example, tool 44 is designed to introduce controlled turbulence to the flow pattern of the slurry 82 which, in turn, creates zones of high proppant concentration and zones of low proppant concentration in the slurry flow, as illustrated in FIGS. 11 and 12. As with previously described embodiments, these heterogeneities are created downhole and transported directly through the perforations 58 for distribution into fracture 56.

By way of example, the bottom hole tool 44 may be coupled to a bottom end of the slurry line 62, and a plurality of propeller devices 102, having propellers 104, may be deployed within the tubing string 34, e.g. within the slurry line 62. The propeller devices 102 are positioned apart from each other at different angles with respect to a vertical axis along slurry line 62. The propellers 104 may be rotated in opposite directions at different angles to induce turbulence to the flow of slurry 82 in tubing 62. As a result, zones with different concentrations of proppant 48 are created in the flow and this leads to creation of heterogeneous proppant structures 52. The proppant structures 52 continued to move with the flow out through perforations 58 and into the fracture 56. Propellers 104 may be powered by power sources or by the energy of the flowing slurry.

Different types of proppant structure heterogeneities may be achieved by, for example, varying the geometry of tool 44. Additionally, the propeller devices 102 may be combined with other components, such as complex pathways and proppant accumulation regions to facilitate creation of the controlled turbulence and structures 52. Additionally, the propellers 104 may have different numbers of vanes and vane configurations, may be rotated at different speeds, and may be started and stopped according to predetermined schedules. The distance between propellers 104 and the relative angles of the propellers 104 also can be adjusted to affect the creation of heterogeneous proppant structures 52. The properties of the slurry flow, e.g. flow rate, proppant concentration, and concentration profile, also may be used to control the concentration of proppant 48 into the heterogeneous proppant structures 52.

Referring generally to FIG. 13, the downhole concentration of proppant 48 into heterogeneous proppant structures 52 also may be accomplished by designing tool 44 to create pressure pulses 106 having given pressure amplitudes and frequencies in the homogeneous flow of slurry 82. The pressure pulses 106 may be created by pulse generators 108, and the amplitudes and frequencies may be changed while flowing the slurry. The pressure pulses 106 may be designed to create standing pressure waves and also may be designed to generate heterogeneous pressure distribution along the slurry flow. The heterogeneous pressure distribution leads to heterogeneous flow disturbance, heterogeneous density and velocity distributions, and ultimately to the creation of heterogeneous proppant structures 52 for delivery into fracture 56.

In this embodiment, the tool 44 may again be constructed in a variety of configurations to provide different ways of generating proppant structure heterogeneities. For example, tool 44 may utilize different forms of pulse generations or different types of pressure amplitudes and/or pressure pulse frequencies. The tool 44 may contain rotating or vibrating parts, different types of discharge nozzles, different numbers of discharge outlets, and other variations in configuration. Additionally, properties of the slurry 82 may be adjusted to achieve varying effects.

Referring generally to FIG. 14, a related embodiment of tool 44 is illustrated in which heterogeneities are created downhole by inducing pressure changes locally. For example, pressure changes may be induced such that localized pressure downhole becomes lower than the vapor pressure of the surrounding fluid. As a result, created cavitations lead to local proppant agglomeration and ultimately to creation of the heterogeneous proppant structures 52. As with other embodiments of tool 44, the heterogeneous proppant structures 52 are created downhole and may be immediately transported through perforations 58 into fracture(s) 56.

By way of example, the tool 44 may comprise ultrasound wave generators 110 which generate ultrasound waves 112. The ultrasound radiation is of sufficient intensity to induce the local pressure changes such that the local pressure is less than the vapor pressure in the surrounding fluid, thus leading to heterogeneous proppant agglomeration. Depending on the application, the number and configuration of the ultrasound radiation generators 110 may be changed. Additionally, various parameters of the ultrasound waves may be adjusted, including frequencies, amplitudes, and other parameters that lead to stationary ultrasound waves. Furthermore, local cavitations may be created in the fluid by combining other methods to facilitate heterogeneous proppant distribution. For example, propellers, tortuous flow paths, accumulation mechanisms, and/or other devices may be combined with the ultrasound wave generators. Parameters of the slurry 82 also can affect creation of the heterogeneous proppant structures 52 in a variety of predetermined sizes and forms.

Referring generally to FIG. 15, another embodiment of tool 44 is illustrated in which heterogeneities are created downhole by inducing rapid temperature changes locally downhole. For example, temperature changes may be induced in tool 44 such that localized temperature increases in the surrounding slurry 82 have sufficient power to create gaseous cavities or bubbles in the fluid. As a result, the created gaseous cavities or bubbles lead to proppant agglomeration and ultimately to creation of the heterogeneous proppant structures 52. As with other embodiments of tool 44, the heterogeneous proppant structures 52 are created downhole and may be immediately transported through perforations 58 into fracture(s) 56.

In this example, the tool 44 may comprise electromagnetic radiation generators 114 which generate electromagnetic radiation 116 capable of rapidly creating heat energy. For example, microwaves may be used to provide high frequency electromagnetic waves which cause localized heating of the surrounding slurry 82. The microwaves induce creation of gaseous cavities in the slurry 82 by this localized and rapid temperature increase, thus leading to heterogeneous proppant agglomeration. Depending on the application, the number and configuration of the electromagnetic radiation generators 114 may be changed. Additionally, various parameters of the electromagnetic radiation may be adjusted, e.g. adjustment of frequencies and amplitudes or the use of ultrahigh frequency and extremely high frequency electromagnetic waves. Parameters of the slurry 82 also can affect creation of the heterogeneous proppant structures 52 in a variety of predetermined sizes and forms.

Referring generally to FIGS. 16-17, another embodiment of tool 44 is illustrated. In this embodiment, fracturing fluid 48, 50 is delivered along a tubing string 34 via at least two flow paths 36. For example, the fracturing fluid may be delivered through internal tubing 40, e.g. slurry line 62, and through an annulus 118 between tubing 40 and the surrounding casing 80. For example, a more concentrated slurry may be delivered along an interior of tubing 40 and a fluid less concentrated with proppant 48, e.g. a clean fluid, may be delivered through annulus 118. In this example, tool 44 comprises a centrifugal mechanism 120 which may be designed to circulate the slurry received from tubing 40. The tool 44 receives the slurry from tubing 40 and controls the fluid discharge speed and direction in space to decompose the slurry flow into at least two flows. The at least two flows of fluid exit through outlets 122. In some applications, the tool 44 may be rotated to create a spiral slurry string 78 (as also illustrated in FIG. 3) which is delivered to the proximate perforation zone. The slurry string 78 is divided into heterogeneous proppant structures 52 as the material passes through perforations 58. As a result, the heterogeneous proppant structures 52 are placed in the fracture 56 heterogeneously.

In some applications, the embodiment illustrated in FIGS. 16 and 17 may be adapted to receive the entire proppant 48 and proppant carrier fluid 50 along one flow path 36. In such application, tool 44 may comprise an additional separation component, such as hydrocyclone 60 illustrated in FIG. 2, to facilitate separation of the slurry into two separate flows of relatively high proppant concentration and relatively low proppant concentration, respectively. In such an example, the centrifugal mechanism 120 may be coupled with the outlet port of the hydrocyclone 60 (or other suitable tool component) through which the higher concentration proppant stream is discharged.

Centrifugal mechanism 120 may have a variety of configurations, including a design in the form of a rotating cylinder 123 connected to a bottom end of tubing 40/slurry line 62. The slurry 82 enters at the top of the rotating cylinder, as represented by arrows 124 in FIG. 17, and flows out through outlets 122 as the centrifugal mechanism 120 is rotated about axis 126. At least two outlets 122 may be positioned to extend from cylinder 123 generally at a lower end of the cylinder 123. The outlets 122 decompose the flow into at least two flows and direct the flows at a tangent to a surface of the cylinder 123. The motion of the flow may be used to create the energy for rotating cylinder 123 and for creating spiral slurry strings 78. It should be noted that centrifugal mechanism 120 may comprise a variety of components, including a variety of nozzles, vanes, blades, rotating parts with nozzles, fixed parts with nozzles, various numbers of nozzles and nozzle configurations. Additionally, the rotation may be supplied by a separate power source, e.g. a motor, or by the energy of the flowing fluid.

Referring generally to FIGS. 18 and 19, another embodiment of tool 44 is illustrated in which flow discontinuities are created by periodic opening and closing of the slurry line 62. In this example, a clean fluid (e.g. a fluid having a lower concentration of proppant 48 or having no proppant 48 at all) is delivered down along a second flow path 36, such as a flow path between slurry line 62 and the surrounding casing 80. The opening and closing of the slurry line 62 at tool 44 creates heterogeneous proppant structures 52 which are carried by the clean fluid flowing along the flow path 36 between slurry line 62 and the surrounding casing 80. The clean fluid carries the heterogeneous proppant structure 52 out through perforations 58 and into fracture 56. In this embodiment, tool 44 also may comprise an additional separation component or components, e.g. hydrocyclone 60, to enhance separation and/or to allow flow of the entire proppant 48 and proppant carrier fluid 50 along one flow path 36.

By way of example, the periodic opening and closing may be achieved by placing a rotatable disc or discs 128 at an end of the slurry line 62. The discs 128 comprise holes or nozzles 130 which are rotated in opposite directions, as indicated by arrows 132 in FIG. 19. The rotation of the discs 128 can be generated by a power source or by the energy of the flow. The rotation of holes 130 past one another creates periodic opening and closing of the slurry line 62 which provides controlled slurry flow discontinuities and ultimately creates heterogeneous proppant structures 52 which may be carried by the clean fluid flow.

It should be noted that tool 44 and rotating discs 128 may comprise a variety of components, including a variety of gates, holes, nozzles, vanes, blades, rotating parts with nozzles, fixed parts with nozzles, various numbers of nozzles and nozzle configurations. For example, the rotating discs 128 may be in the form of cylinders with vertical open slots in each cylinder. Additionally, the rotation may be supplied by a separate power source, e.g. a motor, or by the energy of the flowing fluid. Additionally, slurry and proppant parameters may be varied, e.g. flow speed variations in each flow path, changes in slurry density, changes in proppant concentration, changes in configuration and number of gates, discs and nozzles, and changes in angular velocity and direction of rotation. The tool 44 also may comprise mechanisms to compensate for the pressure increases in the slurry line 62 during stopping and starting of the slurry flow to cause the flow discontinuities.

Referring generally to FIG. 20, another embodiment of tool 44 is illustrated in which at least two distinct flow paths 36 are employed for delivering proppant 48 and proppant fluid 50 to the perforations zone. By way of example, slurry with a higher concentration of proppant 48 may be delivered along an interior of the slurry line 62 while a clean fluid is flowed along the surrounding annulus. In this example, tool 44 is a bottom hole tool coupled to a lower end of the slurry line 62 and injects heterogeneous proppant structures 52 into the flowing clean fluid for delivery through perforations 58 into fracture 56. The generation of heterogeneous proppant structures 52 is achieved based on combination of surface tension forces and clean fluid flow drag forces. The speed and viscosity of the slurry and of the clean fluid are different, e.g. the speed of the slurry flow is less than the speed of the clean fluid flow. Additionally, the viscosity of the slurry is higher than the viscosity of the clean fluid. The bottom hole tool 44 comprises a discharge port 134 sized to create a slurry bubble 136 which grows in size, volume and mass until the clean fluid flow causes the slurry bubble to release and form at least one heterogeneous proppant structure 52 which is transported by the clean fluid through perforations 58. The creation of slurry bubbles 136 is continually repeated to create multiple heterogeneous proppant structures 52.

In some applications, the entire flow of proppant 48 and proppant carrier fluid 50 is directed along one flow path 36. In such application, tool 44 may comprise an additional separation component, such as the hydrocyclone 60 illustrated in FIG. 2, to facilitate downhole separation of the slurry into two separate flows of relatively high proppant concentration and relatively low proppant concentration, respectively. In such an example, the discharge port 134 may be positioned downstream of the outlet port of the hydrocyclone 60 (or other suitable tool component) through which the higher concentration proppant stream is discharged. Additionally, the slurry pipe 62 may be moved in an oscillating or orbiting motion within the casing 80 to facilitate formation of the heterogeneous proppant structures 52 with or without discharge port 134.

In this latter embodiment, tool 44 may again comprise a variety of components, including a variety of tool geometries and release port configurations. Additionally, slurry and proppant parameters may be varied, and such variations may comprise changing flow rate, changing flow rate pulsations, changing density, linear velocity through the discharge port, combining chemical additives, and other variations. Similarly, the clean fluid may comprise a variety of materials and may be delivered at different flow rates, flow rate pulsations, density, and chemical compositions.

In some applications, the tool 44 is designed with a set of features 138, e.g. vanes, disposed along the tubing 40, e.g. along slurry line 62. The features 138 are disposed along the inside and/or outside of the tubular 40 such that the flowing fluid is induced to rotate while moving down toward well zone 55. The rotation tends to concentrate the proppant 48 toward the wall of the tubing. At a suitable location, the width of the features/vanes 138 is increased to separate the layer of dense fluid into spiral stripes. The spiral stripes are released proximate perforations 58 to produce the heterogeneous proppant structure 52. In some applications, the features/vanes 138 may include a generally straight section proximate the exit to reduce remixing of the proppant 48. In these applications, port 134 may not be substantially constricted relative to the diameter of the tubing 40.

Sometimes a pulsation of the proppant 48 is used in creating proppant structures 52 along a longer (or the whole) region containing the plurality of perforations 58. The pulsation may be created by utilizing a cylindrical rotating head 140 (see FIG. 21) attached to the end of internal tubing in, for example, a coaxial arrangement. The rotating head 140 delivers a stream or streams 142 of higher concentration proppant fluid and the stream 142 may be discharged to form a spiral 144 (see FIG. 22) inside casing 80. In some applications, the height of the rotating head 140 may be similar to the length of the region containing perforations 58 which extend through casing 80 and into the surrounding formation 54. However some applications may utilize a rotating head having a height greater or lesser than the length of the region containing perforations 58. Individual or plural slot openings 146 are oriented in a generally circumferential direction and extend generally parallel with an axis 148 of the rotating head 140.

Fracturing fluid of relatively high proppant concentration passes through slot openings 146 and forms a higher concentration vortex within a lower proppant concentration fluid 150 pumped down through the well. The higher and lower concentration fluids are pumped continuously so that proppant is transported radially outward from the rotating head 140. As the streams of higher concentration proppant fluid 142 rotate along spiral 144 into perforations 58 through casing 80, a periodic oscillation is created at the perforations 58. This oscillating concentration of proppant material is pumped into the fractures 56 as proppant structures 52.

The width of the slot openings 146 may be constant on the surface of the rotating cylinder head 140, or their width may be wider on the bottom than on the top to help compensate for frictional pressure drop and to help deliver a depth-independent flux of higher proppant concentration fluid into the created vortex. The rotation rate of the head 140 is associated with the frequency of oscillation of the proppant concentration in the fractures 56. Accordingly, the rotation rate of rotating head 140 may be controlled by, for example, a motor to produce predetermined results. In other embodiments, a propeller may be attached to an outer surface of the rotating head 140 so the lower proppant concentration fluid 150 may be used to control the rotation rate of the rotating head 140. A propeller-like structure also may be located at an internal wall of the rotating head 140 such that the pumping rate of the higher proppant concentration stream 142 controls the rotation rate of the head 140. In other applications, the rotating head 140 may be designed so that the slots 146 direct the streams of higher concentration proppant in a tangential direction to the outer surface of the head 140 to create inertial forces which rotate the head 140.

As illustrated in FIG. 23, the spiraling concentration distribution leads to an oscillating proppant concentration in a radial direction fracture. The oscillation caused by perforations 58 acting in concert with higher concentration proppant streams 142 and lower proppant concentration fluid 150 creates proppant structures 52 in the fractures 56 extending into surrounding formation 54. In this example, the proppant structures 52 are generally created as concentric regions 152, e.g. rings, of higher proppant concentration separated by lower concentration regions 154, e.g. rings, extending radially outward into the fractures 56. It should be noted that the rotating head 140 can be used to discharge streams of low proppant concentration fluid while the higher proppant concentration fluid is pumped down through the well external to the tubing and rotating head 140.

The orientation of the pattern of proppant concentration oscillation has an impact on the fluid production rate coming from the fracture 56. The oscillation makes the proppant placement not only inhomogeneous but also anisotropic. Consequently, the conductivity of such proppant-loaded fractures may be direction dependent. In the example illustrated in FIG. 23, the higher proppant concentration sections 152 are generally parallel with the casing 80 and the fracture 56 has higher conductivity in a direction generally parallel to the casing. In some applications, the higher conductivity direction may be changed through, for example, an anisotropic placement of proppant such that conductivity is higher in the direction generally perpendicular to the casing 80.

Referring generally to FIG. 24, an example of an additional portion 155 of tool 44 is illustrated. The additional portion 155 comprises an internal screw 156 located to affect fluid flow of proppant laden fluid. In the example illustrated, the screw 156 does not utilize moving components but it forces pumped, proppant laden homogeneous fracturing fluid to rotate, as indicated by arrows 158. In this example, the axis of rotation may coincide with the axis of the well. As the pumped, fracturing fluid moves downwardly, it rotates simultaneously and this rotation generates a centrifugal force. The centrifugal force enriches the fracturing fluid with high density proppant particles in the radially outward region while the proppant concentration of the fluid becomes lower along a center, open region 160 of screw 156. The separation is based generally on the same principle as cyclonic separation. However, in cyclonic separators the final direction of the low particle concentration fluid is altered and moved opposite to the gravitational force while the higher particle concentration fluid stream is moved in the direction of gravitational force. In the embodiment illustrated in FIG. 24, however, both the high and low proppant concentration streams move downward toward the region of perforations 58 while rotating, as illustrated in FIG. 25. The high and low proppant concentration streams are indicated in FIG. 25 by the low proppant concentration stream 162 and the high proppant concentration stream 164 moving downwardly within casing 80 toward the perforations.

Referring generally to FIG. 26, another embodiment of at least a portion of tool 44 is illustrated in which a plurality of tubes 166 is attached to some of the perforations 58. In this embodiment, the length of tubes 166 is less than the internal radius of the well. As illustrated, other perforations 58 do not include tubes 166. The arrangement enables creation of perforation belts 168, 170 arranged sequentially along a desired length of the well, as illustrated in FIG. 27. In this example, the first belts 168 are created by the generally orthodox perforation regions having no tubes 166, and the second belts 170 are created by the regions of perforations which include tubes 166. Because of the high pressure inside the well, the fracturing fluid is pushed through the perforations 58 in both belt regions.

The concentration of proppant becomes higher in the fracture region which is close to the orthodox perforations 58, and the concentration of proppant becomes lower in the belt or fracture regions close to the tubes 166. The orthodox regions and the regions including tubes 166 may be alternated sequentially in a vertical direction along the well so that the proppant concentration is oscillated in the fracture along the vertical direction. This leads to an anisotropic proppant placement in which the conductivity of the proppant filled fracture is higher in a direction generally perpendicular to casing 80. As illustrated in FIG. 27, high proppant concentration regions 152 (proppant structures 52) and low concentration proppant regions 154 extend outwardly from and are generally perpendicular with respect to casing 80. In some environments, the generally perpendicular orientation of regions 152, 154 may lead to hydrocarbon or other fluid production rate increases.

Depending on the well fracturing treatment (or other type of subterranean treatment application) and on the desired function of the treatment, the tool 44 and the overall system may comprise a variety of configurations, systems, and components. For example, the tubing string delivery system may be designed to deliver the proppant and the proppant fluid along an individual flow path or along a plurality of flow paths for combination at the downhole tool. Additionally, each of the embodiments described above may utilize a variety of additional, related, or other components designed to facilitate the generation of heterogeneous proppant structures. Furthermore, the embodiments, or portions of the embodiments, may be used in combination with many types of proppant and proppant carrier fluids. In some applications, a separate stream or streams of clean fluid, e.g. fluid having a lower concentration proppant or having no proppant 48, is routed downhole along a separate path or paths for recombination with the heterogeneous proppant structures. The heterogeneous proppant structures may be transported through a casing into fractures or through perforations formed in an open wellbore wall.

The generation of heterogeneous proppant structures may be employed to improve the conductivity along fractures formed during fracturing operations. However, generation of heterogeneous proppant structures at a subterranean location and the local placement of those heterogeneous proppant structures may be employed in a variety of subterranean operations, including non-well related operations.

Although a few embodiments of the system and methodology have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims. 

What is claimed is:
 1. A method for stimulating a subterranean formation by hydraulic fracturing, comprising: delivering a slurry, comprising a proppant and a proppant carrier fluid, down through a wellbore to a downhole tool; using the downhole tool to generate heterogeneous proppant structures; and delivering the heterogeneous proppant structures into a surrounding formation.
 2. The method as recited in claim 1, wherein delivering comprises delivering the proppant and the proppant carrier fluid to the downhole tool along a single flow path.
 3. The method as recited in claim 1, wherein delivering comprises delivering the proppant and the proppant carrier fluid to the downhole tool along a plurality of flow paths in which at least one of the flow paths carries fluid with a relatively higher concentration of proppant.
 4. The method as recited in claim 1, wherein using the downhole tool comprises deploying the downhole tool in close proximity to a perforation zone of the wellbore communicating with the surrounding formation.
 5. The method as recited in claim 1, wherein using the downhole tool comprises using an active downhole tool.
 6. The method as recited in claim 1, wherein using the downhole tool comprises using a passive downhole tool.
 7. The method as recited in claim 1, further comprising coupling the downhole tool to a bottom of a slurry line.
 8. The method as recited in claim 1, wherein using comprises using the downhole tool to apply centrifugal forces to the proppant and the proppant fluid to create the heterogeneous proppant structures.
 9. The method as recited in claim 1, wherein using comprises using the downhole tool to temporarily accumulate the proppant while allowing the proppant fluid to pass.
 10. The method as recited in claim 1, wherein using comprises using the downhole tool to temporarily block flow of proppant and to subsequently release the blocked proppant.
 11. The method as recited in claim 1, wherein using comprises using the downhole tool to direct the proppant and the proppant fluid along a tortuous path to create the heterogeneous proppant structures.
 12. The method as recited in claim 1, wherein using comprises using the downhole tool to create controlled turbulence with respect to flow of the proppant and the proppant fluid to create the heterogeneous proppant structures.
 13. The method as recited in claim 1, wherein using comprises using the downhole tool to separate the proppant and the proppant fluid into two different flows having different concentrations of proppant.
 14. The method as recited in claim 1, wherein using comprises using the downhole tool to periodically interrupt the flow of proppant to create oscillations that result in the heterogeneous proppant structures.
 15. The method as recited in claim 1, wherein using comprises using the downhole tool to create at least one of heterogeneous pressure and heterogeneous temperature distribution with respect to flow of the proppant and the proppant carrier fluid to create the heterogeneous proppant structures.
 16. The method as recited in claim 1, wherein using comprises using the downhole tool to create slurry bubbles without interrupting the flow of proppant to create the heterogeneous proppant structures.
 17. A system for stimulating a subterranean formation, comprising: a tubing string deployed in a wellbore and along which a slurry, comprising a proppant and a proppant carrier fluid, is delivered downhole to a desired well zone; and a tool positioned downhole to receive the proppant and the proppant carrier fluid delivered down along the tubing string, the tool comprising a mechanism to create heterogeneous proppant structures.
 18. The system as recited in claim 17, wherein the mechanism comprises a centrifugal mechanism to separate the proppant and the proppant carrier fluid into the heterogeneous proppant structures.
 19. The system as recited in claim 17, wherein the mechanism comprises a proppant accumulative mechanism which selectively collects and releases the proppant to create the heterogeneous proppant structures.
 20. The system as recited in claim 17, wherein the mechanism comprises a mechanism which introduces controlled turbulence into flow of the proppant and the proppant carrier fluid to create the heterogeneous proppant structures.
 21. The system as recited in claim 17, wherein the mechanism comprises a mechanism which introduces heterogeneous pressure and/or temperature distribution into flow of the proppant and the proppant carrier fluid to create the heterogeneous proppant structures.
 22. The system as recited in claim 17, wherein the mechanism comprises a mechanism which periodically interrupts the flow of proppant to create the heterogeneous proppant structures.
 23. The system as recited in claim 17, wherein the mechanism comprises a mechanism which creates slurry bubbles without interrupting the flow of proppant to create the heterogeneous proppant structures.
 24. The system as recited in claim 17, wherein the tubing string comprises a single flow path for both the proppant and the proppant carrier fluid.
 25. A method for treating a subterranean formation, comprising: delivering a proppant and a proppant carrier fluid to a subterranean location; at the subterranean location, generating heterogeneities of proppant structures with the proppant and the proppant carrier fluid; and transporting heterogeneous proppant structures into a subterranean formation.
 26. The method as recited in claim 25, wherein generating comprises generating the heterogeneity of proppant structures with a tool mounted at an end of a slurry line in a wellbore.
 27. The method as recited in claim 25, wherein delivering comprises delivering the proppant and the proppant carrier fluid along at least one flow path of a tubing string deployed in a wellbore. 